This invention is generally directed to a process for preparing toner resins, and more specifically, a polymerization process for preparing toner resins containing positive charges thereon, which resins may be useful as a component in a developer composition for developing images in various electrophotographic imaging systems.
The formation and development of images on the surface of photoresponsive members is well known as documented in many prior art patents. The latent image that is formed on the photoresponsive device is usually developed by applying electroscopic or toner materials to the surface followed by transfer of the developed latent image to a suitable substrate and optionally permanently affixing the transferred image to the substrate.
Many methods have been described in the literature for preparing toners including suspension polymerization, emulsion polymerization, dispersion polymerization and the like. In one method the softened resin and pigment are mixed thereby resulting in a uniform dispersion by blending these ingredients in a rubber mill, and then pulverizing the materials in order to form them into small particles. Division of the resin pigment dispersions is accomplished by jet pulverization of the material, and although this technique of toner manufacture has resulted in very excellent toners, it does tend to have certain disadvantages including for example it generally produces a rather wide range of toner particle sizes.
In another method of forming toner particles, there is employed the spray drying technique wherein there is blended a water latex of the desired toner resin with a colorant, followed by spray drying the mixture to the desired particle size. More specifically, the spray drying method consists essentially of atomizing the colorant water latex blend into small droplets, mixing these with a gas and retaining the droplets in suspension in the gas until surface tension forces cause the resin particles in each droplet to coalesce, thus encasing the colorant included in that droplet. In some instances spray dryed toners are not totally satisfactory as it is difficult to completely remove all the solvent, and the solvent which remains on the toner particles adversely effects the triboelectric properties between the toner and carrier, and contributes to the blocking of the toner when such is used in the developing processes.
There is described in U.K. Patent Publication 1,319,815 a process for preparing toners directly from monomers by polymerization of the monomer in toner sized particles containing a colorant. A method of suspension polymerization for causing the formation of toner particles is disclosed in U.S. Pat. No. 3,634,251. In the method as described in the British patent, there is prepared a kneaded oil phase component of one or more liquid resin monomers, coloring material, the polymerization initiator, and a finely divided inorganic dispersion stabilizer, such as a metal powder or inorganic salt or oxide, and a polar resinous additive which is soluble in the monomer. Subsequent to suspension polymerization of the monomer, if required, the finely divided dispersion stabilizer is removed by dissolution in an acid and the polymer particles are removed from the aqueous phase and dryed to produce toner. There are some disadvantages associated with this process including the requirement of a high ratio of inorganic stabilizer, which needs to be removed as it will affect the properties of the toner, and thus the quality of the resulting image to be developed.
There have also been proposed suspension polymerization processes similar to the ones described herein but wherein the use of an inorganic stabilizer is eliminated. Generally this process is accomplished by mixing a monomer, a colorant, and an initiator to form an oil soluble organic phase, dispersing this oil soluble phase in controlled a size between 5 to about 20 microns in a water phase, employing the suspending agent such as polyvinyl alcohol, polymerizing, employing conventional suspension polymerization techniques, followed by introduction of a second monomer which is allowed to defuse into the first monomer, and consequently swells the polymer, introducing a water soluble initiator and heating this reaction to effect polymerization of the second monomer and to form the toner.
One method for preparing toners containing positive charges involves the melt blending of a pigment, a charge control agent, and a resin followed by mechanical attrition. Toners prepared by such methods have some disadvantages including incompatibility of the charge control agent, such as a quaternary ammonium compound, or an amine, with the resin thereby the charge control agent is difficult to uniformly disperse in the toner resulting in particle to particle non-uniformity and a wide range of charge distribution. Further, since the charge control agent may be poorly mixed with the resin, the charge control agent may preferentially transfer or leach to the carrier surface thus shortening developer life. Also, the toner resin prepared by suspension or emulsion polymerization usually contains substantial amounts of the residual wetting agent and stabilizing agent, which seriously affects the triboelectric properties and stability of the developers.